1. Field of the Invention
The present invention relates to monolithic ceramic capacitors. The present invention particularly relates to a monolithic ceramic capacitor including a large number of thin dielectric layers and internal conductors each placed therebetween, wherein the dielectric layers are obtained by firing ceramic green sheets (hereinafter referred to as “ceramic sheets”).
2. Description of the Related Art
In recent years, in electronics industries, monolithic ceramic capacitors including a sintered ceramic body and internal conductors placed therein have been widely used.
In such monolithic ceramic capacitors, ceramic dielectric layers (hereinafter referred to as “dielectric layers”) are each placed between the internal conductors. Since there has been a demand for a decrease in size, increase in capacity, and decrease in cost, new monolithic ceramic capacitors including thin dielectric layers having a thickness of about one μm have been recently developed.
In general, a decrease in thickness of a dielectric layer reduces the number of ceramic particles placed in the thickness direction of the dielectric layer and increases the electric field applied to the dielectric layer; hence, the reliability of the dielectric layer is decreased. Therefore, in order to increase the reliability, the average size of the ceramic particles has been reduced, whereby a new barium titanate dielectric material has been developed. This material is useful for forming dielectric layers having a thickness of about one μm.
For conductive materials for forming internal conductors, a base metal such as Cu or Ni has been used instead of a noble metal such as Ag or Pd, whereby low-cost monolithic ceramic capacitors have been developed.
On the other hand, in order to increase the capacity of a monolithic ceramic capacitor, it is presumed that the dielectric layer thickness must be reduced. However, when the internal conductors are formed by an ordinary screen-printing method, there is a limit on the reduction in thickness of the internal conductors. Accordingly, a layered body including the dielectric layers and internal conductors is apt to warp.
In the layered body, when the number of the dielectric layers is large, a region having the dielectric layers and internal conductors has a thickness larger than that of another region having the dielectric layers but no internal conductors depending on the presence of the internal conductors. Accordingly, the layered body is apt to warp. Thus, the thickness of the internal conductors must be minimized.
Since a conductive paste for forming the internal conductors usually contains a powdery conductive material, an organic binder, and an organic solvent, the green internal conductors have a thickness two to three times larger than that of the fired internal conductors containing the conductive material only.
As described above, there is a limit on the reduction in thickness of the dielectric layers when the internal conductors are formed by a screen-printing method. Therefore, it is difficult to prevent warpage due to the internal conductor thickness from occurring in the layered body, and structural defects such as delaminations and cracks may occur in a step of removing the binder or in a firing step.
In order to cope with the above problem, the following technique is disclosed in Japanese Unexamined Patent Application Publications No. 64-42809 and No. 8-124787 (hereinafter referred to as Patent Documents 1 and 2, respectively). A metal layer is formed on a film by a film-forming process such as a vacuum deposition process or a sputtering process, and the resulting metal layer is then transferred to a ceramic sheet, whereby a thin, dense internal conductor is provided on the ceramic sheet.
In the technique disclosed in Patent Documents 1 and 2, since the internal conductor is formed using a thin film containing only metal, the internal conductor has an extremely small thickness; hence warpage due to the internal conductor thickness can be greatly reduced.
However, in the internal conductor prepared by the above technique, a conductive component contained in the metal film forms aggregates of microcrystals. Therefore, when the internal conductor is fired at 1,200° C. or more, the degree of sintering is excessively increased, whereby the internal conductor is caused to partially shrink and spaces are formed in the internal conductor. Therefore, the area of the substantial part of the internal conductor is decreased.
Thus, when the internal conductor is prepared using a metal film, the metal film must have a sufficient thickness to a certain extent. However, an increase in thickness of the metal film prevents a decrease in thickness of a monolithic ceramic capacitor and causes an increase in manufacturing cost. That is, according to known techniques such as one disclosed in Patent Documents 1 and 2, the dielectric layer can be reduced in thickness but the internal conductor cannot be sufficiently reduced in thickness. Therefore, there is a limit on the number of layers of the monolithic ceramic capacitor, and monolithic ceramic capacitors having a large capacitance greater than or equal to, for example, 100 μF have not been obtained.